Cell-mediated cytotoxic response to syngeneic tumor: By employing the IUdR release assay, primary and secondary cell-mediated cytotoxic response could be generated both by in vivo and in vitro methods, and these immunological reactions were found to give very good correlation to the in vivo immunity. This may allow us to be able to further study the mechanisms for the induction of in vivo immunity. Humoral control of tumor growth and immune response: We have found that humoral factors were produced in the tumor bearing hosts of man and mouse, that could promote the tumor growth and suppress immune response. Further attempts are made to isolate and purify these factors. In addition, antisera and immunoregulatory alpha-globulin were also found to be able to affect the in vivo or in vitro immunological reactions. These studies should help the understanding of the causes for tumor progression, and to develop means to control tumor growth and immune responses. Characterization of soluble antigens: Soluble antigen extracted from FBL-3 cells, a Friend virus-induced leukemia, and gp70 were found to inhibit the cell-mediated cytotoxic reactions to FBL-3 cells, but not by H2 antigen or P30 prepared in similar ways.